This invention relates to a semiconductor laser module used in a directly modulated optical communication system.
In recent years, semiconductor laser modules have been tested intensively with regard to their use in such fields as CATV, public telecommunication, and the handling of micro-wave signals. Their implementation in these fields is already on the way. In short and medium range transmission in the range of several hundred meters to several kilometers, the accumulation of noise and signal distortion is comparatively small. Thus the use of direct intensity modulation in which micro-wave signals are directly modulated and transformed into optical signals is advantageous in view of equipment convenience and costs.
FIG. 16 illustrates an equivalent circuit for a conventional semiconductor laser module, showing a module package 11, a laser diode chip 12 and a transmission line 13 inside the module package, a dc bias input circuit 14, a modulation signal input terminal 15, a transmission line 16 with typical characteristic impedance and an input impedance matching resistance 17.
The semiconductor laser module is operated as follows. First, a dc driving current is drawn from the dc bias input circuit 14 and stimulates emission in the laser diode chip 12. Next, a micro-wave modulation signal from the modulation signal input terminal 15 is directly modulated and converted into an optical signal in the laser diode chip 12. Usually, the impedance of the laser diode chip is several .OMEGA., and the characteristic impedance of the transmission line 16 is 50 .OMEGA.. In this case, matching with the characteristic impedance of the transmission line 16 is achieved by inserting the input impedance matching resistance 17.
However, since in this conventional structure the serially inserted load is several times bigger than the laser diode chip load of several .OMEGA., a large part of the modulation signal power is dissipated by the input impedance matching resistance 17. Therefore, the input power converted by laser diode chip 12 drops and the modulation index becomes small.
FIG. 18 shows the impedance matching expressed by the voltage standing wave ratio (VSWR) for a laser diode chip load of 3 .OMEGA. and a load resistor of 39 .OMEGA.. The optical modulation level, expressed by S21, the ratio of optical modulation output power and modulation signal input current, is shown in FIG. 19. As can be seen from FIG. 18, the VSWR can be made small and impedance matching achieved by inserting input impedance matching resistance 17. However, as can be seen from FIG. 19, the modulation signal power that can be converted by the laser diode chip and the output level is small, because a large part of the power is dissipated at the resistor. Consequently, it becomes necessary to increase the modulation signal power in order to achieve the desired modulation index. As a result, a pre-amplifier must be provided, or the gain of an existing pre-amplifier must be stepped up, thus causing higher complexity, dimensions and costs for the system. Furthermore, higher output of the amplifier will involve an increase in intermodulation distortion.
Another example for a conventional semiconductor laser module is shown in FIG. 17. Matching of the characteristic impedance of the transmission line 16 and the input impedance of the semiconductor laser module in the desired frequency band is realized by inserting an LC-type impedance matching circuit 19 into the input circuit outside the package module 11 and selecting the LC circuit constant accordingly.
However, the transmission line 13, located inside the module package and connecting the laser diode chip 12 with the modulation signal input terminal 15, has an inductance function, because it comprises a micro-strip line and a bonding wire. Therefore, it limits the frequency range in which impedance matching with the LC-type impedance matching circuit 19 is possible. FIG. 20 illustrates the frequency characteristics of impedance matching expressed by the VSWR. FIG. 21 shows the frequency characteristics of the optical modulation level as expressed by S21, the ratio of optical modulation output power to modulation signal input current. The VSWR can be made small by inserting the input impedance matching circuit 19, and since a large amount of modulation signal power can be converted by the laser diode 12, a high optical output level can be realized around 600 MHz, as can be seen from FIG. 20 and 21. However, due to the influence of the electric length of the transmission line 13, which is located inside the module package and has as an inductance function, impedance matching may not be possible for the desired frequency band, e.g. for 1200 MHz.
Moreover, it is necessary to transmit signals in several frequency bands for the optical transmission of radio signals. Yet in this case, matching of the input impedance and efficient modulation of the laser module are even more difficult.
Also, in order to accommodate the LC-type input impedance matching circuit 19 comprising several circuit elements, space has to be provided outside the module package, thus enlarging the apparatus.
Furthermore, regarding the LC-type input impedance matching circuit 19 located outside the module package, variations of the material of the printed circuit boards employed, the transmission lines, the matching circuit elements, and the mounting conditions of module and circuit elements can cause irregularities in the circuit constants that will lead to a decline of certain characteristics, such as a drop of the conversion efficiency or a shift in the matching frequency band. Therefore, the desired frequency characteristics cannot be guaranteed, and examination of the input impedance matching conditions and tuning becomes necessary for each semiconductor laser module device. This can result in rising costs or a dropping yield.